Concentrating and reacting with nitric acid



Oct. 12, 1965 w. A. SMITH ETAL 3,211,525

CONCENTRATING AND REACTING WITH NITRIC ACID 6 Sheets-Sheet 1 Filed Aug.8, 1963 Oct. 12, 1965 w. A. SMITH ETAL 3,211,525

CONCENTRATING AND REACTING WITH NITRIC ACID 6 Sheets-Sheet 2 Filed Aug.V8, 1965 n l www n n IA/ n moz: oo lllllllll KRUE3MAN LET Oct. l2, 1965w. A. SMITH ETAL 3,211,525

CONCENTRATING AND REAGTING WITH NITRIC ACID Filed Aug. 8, 1963 6Sheets-Shee'cI 4 HCL W/LL/AM AUBREV ENTORS F E JOSEPH M DOWNEY l EJOSEPH J. JACOBS NLE Y MAN JOHN D. BUEHLER. FRED D HU/LL T ATTORNEYJOct. l2, 1965 W. A. SMITH ETAL CONCENTRATING AND REACTING WITH NITRICACID Filed Aug. 8, 1963 HNOS HNO3

KNo3

6 Sheets-Sheet 5 HNO:5 i /IO INVENTORS WILL/,4M AUB/25V SMITH JOSEPH M00 JOHN D. BUEHL FRED D. HU/LLET ATTORNEYJ` OC- 12, 1965 w. A. SMITHETAL 3,211,525

CONCENTRATING AND REACTING WITH NITRIC ACID Filed Aug. 8, 1963 6Sheets-Sheet 6 JOSEPH J JACOB FRED o.' H11/LET BY /ff J7, f M JL/Ay.

ATTORNEYS United States Patent O 3,211,525 CONCENTRATING AND REACTINGWITH NITRIC ACID William Aubrey Smith, Pasadena, Calif., Joseph M.Downey, Vicksburg, Miss., Joseph J. Jacobs, Altadeua, Stanley L.Krugman, Pasadena, and John D. Buehler, Whittier, Calif., and Fred D.Huillet, Golden, Colo., assignors to American Metal Climax, Inc., NewYork, N.Y., a corporation of New York Filed Aug. 8, 1963, Ser. No.301,937 30 Claims. (Cl. 23-160) This application is acontinuation-in-part of our copending applications, Serial No. 862,000,filed December 24, 1959, and now abandoned; 74,730 filed December 8,1960, and now abandoned, and 164,792, led January 8, 1962, and nowabandoned.

This invention relates to new and useful improvements in concentratingnitric acid containing dissolved nitrate salts of the metals in group Iahaving an atomic weight at least as heavy as potassium and particularlyfrom a weak to a strong nitric acid solution. The resultant strongnitric acid may be removed as the product, fractionated up to 100%nitric acid or utilized in a process for reacting chlorides ofpotassium, sodium, and/or hydrogen to produce separated or mixednitrates of potassium and sodium, chlorine, and nitrogen dioxide, thelatter being subject to withdrawal as product or recycle to fortifynitric acid in the system. If potassium chloride is used as startingmaterial, at least a portion of the resultant potassium nitrate may beydissolved in said nitric acid during the concentrating process.

The invention also relates to a process wherein the starting chloride isexposed to relatively high concentrations and excess stoichiometricquantities to maintain high concentrations of nitric acid in a strippingzone under conditions permitting complete stripping of chlorine andnitrogen oxide produced, although stripping of chlorine is moreimportant as its presence tends to reverse the desired reaction. Furtherimportant features of the invention are using Weak nitric acid for thelirst portion of the reaction and introducing strong nitric acid foronly the iinal portion of the reaction and dividing the reaction zoneinto a gas reaction area in the absence of the starting chlorides and asolution reaction area with the chlorides.

Throughout this application, the term strong nitric acid refers tonitric acid concentrations with water wherein the Iacid :component isgreater than the normal azeotropic composition, i.e., from above about68% up to 100% HNO3 by weight. Weak nitric acid refers to any acidconcentration with water below the normal azeotropic composition, ie.,below about 68% HNO3 content. The normal water-nitric acid `azeotropecontaining 68% HNO3 or thereabouts at atmospheric pressure is thedividing line, regardless of the actual azeotrope resulting from a givenmixture which may be considerably removed from 68%, e.g., as madepossible by the presence of various nitrates in solution as will behereinafter discussed more fully. The concentrations for purposes ofmeasurement unless otherwise specified, refer only to the nitric acidand water components and are not concerned with any other material, andparticularly nitrates, which may also be present in the mixture.

As is Well known, ordinary distillation and rectification will notproduce strong nitric acid since an azeotrope containing about 68%nitric acid and 32% water is formed, i.e., both the liquid land vaporover a 'boiling solution have the same composition. The addition ofvarious dchydrating agents has been known (e.g., sulfuric acid andmagnesium nitrate) to avoid the normal azeotrope formation and thuspermit formation of a vapor rice mixture containing relatively morenitric acid than otherwise. It would be advantageous to avoid the normalazeotrope formation in such a way as to permit formation of a vaporcontaining relatively more water than otherwise, as water vapors areconsiderably easier to handle and cheaper to evaporate. However, thedehydrating agents may be used in alternate steps in conjunction withthe nitrates of this invention.

It is old to react KCl, NaCl [or HCl to lproduce C12, KNO3 or NaNO3 butthe reaction proceeds very slowly toward completion and also forms NOClwith water vapors which provide serious corrosion problems. The NOCI maybe withdrawn, H2O and HNO3 vapors removed, and oxidized elsewhere toform NO2l (used interchangeably herein with N204) and C12 but this is anexpensive and extra operation. i

Moreover, in the past reactions similar to this have utilized a highgrade chloride starting material. It is very desirable to have a process-th-at can utilize not only high grade chlorides but commercial muriateof potash containing 95 to 97% KCl and up to 5% NaCl plus otherimpurities or sylvinite containing up to about NaCl, 30 to 40% KCl andimpurities, e.g., as mined in the Carlsbad, New Mexico, area of theUnited States or in Saskatchewan, Canada.

If it is desired to use strong HNO3 as has been suggested, this becomesan expensive process, as conventional commercial HNO3 does not go abovethe normal azeotropic composition (68% HNOa).

Although hydrochloric acid is a by-product in many industrial processesand thus generally available at cheap prices, there has not been foundan economi-cally successful process for producing the more valuableelemental chlorine therefrom.

Therefore, it is an object of this invention to increase the HNO3concentration of HNO3-H2O solutions at' any range, but particularly frombelow to above the normal azeotropic composition.

It is also an object of this invention to remove water from HNO3H2Osolutions with or without dchydrating agents.

It is likewise an object of this invention to establish conditionswhereby the reaction of KCl,NaCl and/0r HCl with HNO3 to form KNO3,NaNO3 and/ or C12 -may go readily to completion in a manner to eliminatechlorides in the early stages as far as equipment is concerned so thatexpensive corrosion-resistant equipment is not necessary in the laterstages of the process.

It is an additional object of this invention to optionally producestrong HNO3 within the chloride to chlorine process to be used in thebasic reaction.

It is a further object of this invention to supply as starting materialweak HNO3 in stoichiometric quantities but to have both an excess andstrong HNO3 in the reaction zone.

It is another object of this invention to introduce weak HNO3 into therst portion and -strong HNO3 into the final portion of the reaction.

It is likewise an object of this invention to divide the reaction zoneinto a gas reaction area in the absence of starting chlorides and asolution reaction area with chlorides with the strong HNO3 beingintroduced and partially spent in the gas reaction area before passageto the solution reaction area.

It is an additional object of this invention to utilize readilyavailable muriate of potash or sylvinite as a starting material ifdesired instead of the relatively more expensive high grade KCl or NaCl.

It is a further object of this invention, `when starting with mixedchlorides of potassium and sodium to be able to produce separately thecorresponding nitrates. i

For H+ (hydrochloric It is another object of this invention that thechloride be substantially completely reacted and then recovered as Cl Itis likewise an object of this invention to utilize iron which isnormally present in muriate of potash to mimmize the, corrosiveness ofminor amounts of chloride ions 'thatbe left in the solution afterremoval of the C12.

`It' is"also an object of this invention that all of the nitrogen fromthe used HNO3 ultimately be removed as KNO3, NaNO3 or mixtures if sodesired but may be removed partially as NO2 and/or strong HNO3.

It is an object of this invention to produce elemental chlorine cheaplyfrom hydrochloric acid and in a process and equipment which mayalternatively produce alkali nitrates from alkali chlorides or strongnitric acid.

It is a further object of this invention to produce elemental chlorineby oxidizing hydrochloric acid with nitric acid in such a manner thatonly air (for O2) and hydrochloric acid need be consumed if so desired.

We have found that the nitrates of group Ia metals having an atomicweight at least as heavy as potassium, when dissolved in an aqueoussolution of HNO3 quite unexpectedly permit the production of adistinctly more concentrated HNO3 in the solution by shifting theazeotropic point upwardly for HNO3 (downwardly for H2O), so that theresultant solution may become higher than 68% (conventional HNO3-H2Oazeotrope) HNO3 by mere boiling. Thus this azeotropic change results ina reduced vapor pressure of HNOS with added evaporation of water, theextent depending primarily on the concentration of the nitrate used butpartly on the particular .and all or part of the strong acid recycled inthe basic process for driving the reaction eiectively to completion.Consideration of the following reactions (in which Na may be substitutedfor K) are necessary to properly understand the overall chloride tochlorine process:

For K+ (potassium or sodium chloride) IFor both K+ and H+ course,results in differences in the cumulative reactions (I) and (A). Thisdifference is simply the addition of '1 mole of HNO3 per mole of KCl toprovide NO3- for the recovered KNO3. In regard to the KCl reactions, re-

action (I) is the ultimate reaction obtainable in view of the fact thatthere is exhibited a 1:1:1 mole ratio between K+, NO3- and Clon bothsides of the equation, whereas reaction (A) requires 2 moles of startingNO3+ per each one produced on the right and reaction (1) requires 4moles of starting NO3- and 3 moles of starting Cl per 3 and 2,respectively, produced in desired form on the right. In regard to theHC1 reaction, reaction (I) is the ultimate result obtainable, utilizingonly O2 (from the air) for the HC1 oxidation but the O2 is actuallyconsumed indirectly in regenerating NO2 (used interchangeably hereinwith N204) from NO.

We have developed an overall process as described by reactions (I),i.e., the ultimate theoretical effectiveness obtainable in convertingKCl and/or NaCl and HNOS to KNO3 and C12, even when using sylvinite ormuriate of potash or the oxidation of HCl merely with O2 from the airwith the aid of strong HNO3 that need not be ultimately consumed.However, reaction (B) occurs outside the primary reaction zone and isused only if the NO2 is not otherwise disposed of, e.g., as product perse. Reaction (A) may be used without (B) and (A) is driven to completionin the primary reaction zone, if proper conditions are utilized, andparticularly if suicient excess stoichiometric HNO3 is used to eectabout 40-65% HNO3 in H2O at the bottom of the reaction zone afterreaction with the chloride (or muriate of potash) and dilution withreaction H2O. A 5460% acid concentration range in the bottoms has beenfound extremely advantageous but it can be reduced easily to about 50%and yet be commercially attractive. With less advantageous results froman economic and design standpoint, the range may extend from about 40 to65%.

The reaction with HCl, in the absence of any nitrates or impurities suchas iron that may be picked up in- KCl and NaCl, preferably operates with57-60% HNO3 concentration at the end because of corrosion problems.

The reaction of NaCl with HNO3 is faster and easier to drive tocompletion than KCl (and muriate of potash is even slower than technicalKCl). Thus the excess stoichiometric quantities may be such to yield aconcentration of HNOS at the reaction end of 30 to 40% for NaCl.However, concentrations and quantities should remain the same in the gasreaction area since no Na+, K+, or Clis present there. Moreover, thesolubility of NaNO3 relative to KNOB is considerably less so that excessof acid and/or water may be necessary to hold the NaNO3 in solution. Aswill be noted in this paragraph and also hereinafter, many properties ofthe sodium compounds differ surprisingly from the potassium compounds,leading to their easy separation herein if both are present and t0surprising results from the effect of KNO3 which markedly increases theHNO3 content of the azeotropic composition of HNO3-H2O, whereas NaNO3lowers the HNO3 content slightly. It is preferred to add the outsidesource acid (stoichiometric quantity plus nominal processing losses) tothe starting chloride material with at least a part of the acidsufficiently cooled to give a slurry having a temperature below about 60as there is practically no reaction at or below this temperature level.Thus the slurry can be introduced into the closed system without givingoi fumes that are harmful and disagreeable to personnel and equipment.Once the slurry is within the closed system, the heat will be raised tothe appropriate level to support the reaction.

We have found it most advantageous to react for a relatively shortperiod of time the chlorides and commercial HNO3, in sucient quantitiesand heat to dissolve the chlorides during which the reaction willproceed roughly from 40 to 80% completion without further aid.Thereafter, however, the reaction is advantageously carried out undersuch conditions so as to strip the reaction gases from the solution.Thus the. sQlutiqn iutrocluced into a second reaction area along withstrong nitric acid where it is stripped of reaction gases.

The strong HNOS is formed in the process by evaporation of Weak HNOScontaining dissolved KNOS, the latter changing the azeotropiccomposition. Evaporation is utilized also to recover KNOS and/or NaNOScrystals so that the mother liquor becomes strong HNOS. Once the motherliquor has become strong HNOS, it may be recycled as such or preferablyas clean HNOS by distillation of vapors from the strong mother liquor.It is important that this second reaction area have separate zones,namely, a gas reaction zone where the off-gases may be reacted with theclean most concentrated HNOS to drive reaction (2) to completion(substantially eliminate NOCl from the system) and a liquid-reactionzone where the partially spent HNOS then reacts with the chloridesolution.

We have found that the second reaction area should be carried out in atower (preferably bubble plate co1- umn) to increase retention time withoff-gases moving countercurrent to HNOS and the chloride solution. Withthis area divided into the two described zones, the countercurrentmovement of the chloride solution only occupies a bottom portion of thecolumn but HNOS moves throughout the entire length. Although a tower ispreferred, any device which provides retention and countercurrentmovement of the reactants could be used.

It will be appreciated that strong HNOS, i.e., between 68 and 100%, maybe produced as a by-product or main product of the processes herein. Inview of the peculiar effect of certain nitrates on the azeotrope, itwill be necessary when producing strong HNOS without KCl as a startingmaterial, i.e., starting with NaCl or HC1 only or operating Withoutchlorides, to incorporate one of the nitrate salts of the metals inGroup la having an atomic weight at least as heavy as potassium in themother liquor circuit in order to produce strong HNOS.

With the above objects and features in view, the nature of which will bemore apparent, the invention will be more fully understood by referenceto the drawings, the accompanying detailed description and the appendedclaims.

In the drawings:

FIG. l is a diagrammatic flow sheet for conversion of KCl, HNOS andminor quantities of NaCl to KNOS, NaNOS and C12 in accordance with thisinvention (gas flows are shown in double lines and liquor or solids insingle lines throughout);

FIG. 2 is a diagrammatic flow sheet when starting with substantialquantities of both KCl and NaCl;

FIG. 3 is a graph showing the solubility of KNOS and NaNOS in thesaturated presence of each other at various temperatures andconcentrations of HNOS;

FIG. 4 is a graph showing the effect of varying amounts of KNOS aloneand with NaNOS on the azeotropic composition of HNOS and H2O;

FIG. 5 is a diagrammatic ow sheet for oxidation of HC1 to C12 with HNOS(modifications shown in dot or dash lines);

FIG. 6 is a diagrammatic flow sheet showing the concentrating of HNOS;

FIG. 7 is a diagrammatic flow sheet showing the concentrating of HNOS bycombining alternately steps of thisA invention and prior art steps;

FIG. 8 is a diagrammatic flow sheet showing a modified combination ofalternate steps; and

FIG. 9 is a graph showing the effect of various nitrates in solution onthe azeotropic point of HNOS-H2O'.

This invention as particularly illustrated in the drawings disclosescomplete processes for the production of industrial or agriculturalgrade KNOS, NaNOS, C12, NO2 and strong HNOS, i.e., a higherconcentration than constant boiling or normal iazeotrope HNOS of about68- v 69%. It will be appreciated, of course, as explained herein and.as obvious to those skilled in the art, that various 6 subprocesses ofthis complete process described may be practiced independently andfurther that there are many variable conditions that may be changedthroughout the process by reason of different grades of startingmaterials available, end products desired, or various other factorsdictated by economy, demand, etc.

FIG. l FLOW Crude muriate of potash (approximately 96% KCl, 2.6 NaCl,0.3 MgCl2, MgSO4, 0.17 polyhalite, 0.05 FeSOS, 0.10 A1203, 0.23 acidinsoluble and 0.25 H2O) is stored in bin 6. Fresh weak HNOS of 55%concentration is stored in tank 7. Due to the fact that strong acid isproduced in the system, weak acid of any strength may be .the outsidesource of acid. In fact, the weak outside acid may also be fortified byNO2 produced in the system. The acid is refrigerated at 10 and thensuicient quantity fed into the pulping tank 8 with potash to form aslurry up to about 70% solids which makes it convenient for furtherfeeding control of the combined starting materials. The pulping tank ismaintained at slightly negative pressure and below 60 F., preferablyabout 30 F. (all temperatures herein are Fahrenheit and pressures areatmospheric unless otherwise indicated) so as to substantially avoidreaction at this point. The tempera-ture is controlled largely by therefrigerat-ion of the acid.

The resultant slurry from pulping tank 8 is then fed into steam jacketedsolution tank 9 where additional acid without refrigeration is fed fromstorage 7 (or other stages of the process), the total acid beingsufficient so that substantially all solids go into solution. Theternperature is maintained at about 150. Under these conditions about50% of the reaction takes place when the solids have been dissolved andit is preferred that the remainder of the reaction take place in thecolumn, although 40 to 80% may be conveniently permitted in solutiontank 9. The solution from tank 9 is then fed intermediate the ends intocolumn reactor 11, which has several spaced trays. This tray columnprovides an extended period of time, eg., 5 minutes for passage of theliquid down through the column which permits the reaction to gosubstantially to completion with a resultant complete stripping of theC12 (and other gases) from the solution in the reactor. Thus allchlorides are removed in the column reactor except for the retention ofan iron chloride complex as ia contaminating factor but as will beexplained hereinafter, for practical purposes, the processing of theprimary liquid strea-m from this point on is generally free of thecorrosion that normally accompanies chlorides in admixture with HNOS.

The reaction mix or bottom liquid from tank 9 is in troduced at fanintermediate point 12 of column reactor 11. The reaction gases from tank9 are introduced slightly above the liquids at point 13 which serves todivide column reactor 11 into an upper gas reaction zone 14 and a lowerliquid reaction zone 16. Clean liquid HNOS of about 75% concentration isintroduced near the bottom of the gas reaction zone, HNOS vaporsintermediate the ends and 85 clean liquid HNOS slightly above thevapors, all of which serve to oxidize any NOCI to NO2 and C12 in`accordance with reaction (2). Some of the 85% liquid may be introducediat the top of the column as reflux. The acid (somewhat more dilute)will then, of course, pass into the liquid reaction zone 16 to reactwith KCl. It is preferred to use the acid at the higher concentrationfor reaction (2) and then at the resulting lower concentration forreaction (l) in the liquid reaction zone 16. To insure completion ofreaction (2) in the gas reaction zone, it is necessary to use strongHNOS and preferably in sufficient excess to have `strong acid remainingat the bottom of the gas reaction zone. Strong acid is shown enteringzone 14 at three places which is desirable but not necessary. Othercombinations and more or less entries may be used. In fact, the acidwill be more concentrated toward the top of the zone [assumingsufficient excess present to overcome spending and dilulosses.

(the boiling temperature of 85 to 100% HNO3). Vthese specifiedtemperatures and concentration and quan- -tity of acid, the reactiongoes to completion and only tion of acid by reaction (2)] when thehighest entry plate has strong acid on it because the vapors from astrong acid solution are even more concentrated in HNO3 and thus theseries of plates and ultimately the condenser 17 above which refluxesthe acid will give a stronger acid which will be 100% if there aresufficient plates. Conveniently, the HNO3 concentration may be about 95%at the top and 70% at the bottom of zone 14.

For convenience, the upper trays of the gas reaction zone may be spacedabove and separate from the lower trays of the liquid reaction zone aslong as liquids pass from zone 14 into zone 16 and gases pass from zone16 into zone 14. Actually as far as the reaction is concerned, solutiontank 9 is part of the l-iquid reaction zone 16 and could be eliminated.However, it is preferred and more economical to complete lat least about40% of the reaction .and to get solids into solution in the cheapersolution ftank equipment rather than the expensive tower equipment. Theprovision of separate zones within the reaction column by introductionof KCl intermediate the column ends is an important feature of thisinvention and has the advantage of carrying out reaction (A) in onecolumn or area and yet having reaction (2) carried out `in a zone apartfrom the liquid reaction and with the strongest HNO3 available to insurecomplete conversion of NOCl to NO2 and C12.

There is present in the entire reaction area sufficient stoichiometricquantity of acid necessary to complete the reaction and to maintain acidconcentration preferably at 5460% (for KCl) in the column bottoms,despite the `spending and dilution thereof with the production ofreaction water. The excess acid is provided and controlled by the rateof recycle, the net consumption of acid being only stoichiometricquantities, plus nominal processing A relative high concentration ofHNO2 is made possible by the recycle of acid mother liquor with KNO3 insolution which results in nitric acid at concentrations above theconstant boiling 68-69% normally obtained. In fact, this recycle motherliquor when saturated with KNO3 would appear to reach its azeotrope atabove 90% rIINO'depending on KNO3V concentration and pressure.

The acid mother liquor may be recycled per se but we prefer to recycleit at least in part as clean strong acid by vapor formation andcondensation to thus avoid buildup of impurities in the reaction areaand also to obtain, if desired, a stronger acid (by fractionation) thaneven the strong acid mother liquor.

The solution is fed to the column reactor at about 150 and the reboilerat the bottom of the reactor maintains a temperature of about 320, withthe off-gases coming off the top of the gas reaction zone at about 130to 200 At NO2 and C12 come over in substantial quantity in the offgases,the NOCl having been completely oxidized to the other two gases, by theexcess HNO3 in accordance with reaction (2).

The off-gases from the gas reaction zone, however, contain vapors ofHNO3 and H2O (possibly a trace only of `NOCI) and are thus verycorrosive.

Accordingly, they are passed to the partial condenser 17 operating at atemperature of below about 70, preferably 45 to 50, to remove thelsevapors. Alternatively a direct contact y Chiller column may be used tocountercurrently scrub the loff-gases with cold HNO3.

From the partial condenser 17 the dry gases are passed l to a condenser18 at about 35 and about 20 p.s.i.a. and vthen the liquid is passed to afractionating tower 19 operating at about 55 p.s.i.a., with an overheadtemperature of about 30 and a bottom temperature of about 122. The

. C12 overhead product is passed to a condenser (not shown) and then tostorage as better than 99.9% C12.

The recovery of C12 in the process is easily better than 97% ofth-eoretical yield. In order to readily produce high purity about 155p.s.i.a.

overhead product, the NO2 bottoms intentionally contain a few percent ofC12 along with the NO2 and any trace of NOCl in the feed. The bottomsare pumped to the NO2 fractionator 21 which operates at about 55p.s.i.a., with an overhead temperature of about 45 and a bottomtemperature of about 124. Pure NO2 (99.99%) is the product offractionator 21 withdrawn through vent 51 which is spaced sufficientlyabove the column bottom to be in the vapor zone and thus avoid any HNO3that may be in the condensed bottoms. A few percent NO2 are taken withthe overhead C12 and NOCl in order to insure a high purity bottoms andall are recycled to the bottom of the upper zone 14 of the columnreactor 11 along with a bleed portion from the column 21 bottoms.

The NO2 product may be condensed at 52, be taken as an end product forvarious commercial uses or it may be taken to a standard HNO2 absorptiontower 22. Demineralized water is fed from receptacle 23 to the top, theNO2 at the bottom, and fresh 55% HNO3 from tank 24 is fed intermediatethe ends but nearer the top. This 55 acid is the outside source feedacid, the concentration being that which is readily available on acommercial basis. However, any strength acid or only H2O could be usedat this point. The tower 22 operates at Air for oxidation of NO2 issupplied near the bottom by a compressor. Demineralized water is usedfor absorption at the top tray where the gas is lean in NO2, and the 55%acid is fed on the appropriate tray. The resultant 65% acid product canbe used in the process whenever clean weak acid is desired, particularlyat tanks 40 through gas reaction zone 14, by using less concentratedand/ or lesser quantities of HNOS in zone 14, or by making zone 14relatively smaller (or eliminating) in comparison to liquid reactionzone 16.

Liquor is withdrawn from the bottom of column re- 45 actor 11 largely asa solution of KNO?I in about 58% HNO2 substantially free of chlorideexcept for a minor amount tied up in an iron chloride complex which isnot particularly harmful from a corrosive standpoint. Thus from thispoint on in the process the normally corrosive 50 chlorides are oflittle concern and it is'possible to consider stainless steel or similarequipment in the designing rather than glass lined or similar equipmentthat is normally required to this point, although hot strong HNO2 ispresent downstream and must be considered in choice of materials anddesign (particularly temperatures and pressures) from the standpoint ofcorrosion.

It was thought that the iron present in commercial sylvite would createa contamination problem for a recycle process. Moreover, it was knownthat ionized chloride must be kept at a minimum beyond the columnreactor if expensive equipment is to be avoided. It was found that arelatively stable (i.e., not ionized) and soluble iron chloride complexis formed. Iron other than the complex is soluble t-o at least 0.6% ironso that this amount of iron can be tolerated, and in fact, appears toaid the chloride problem at certain temperatures and acidconcentrations. The chloride ion maximum is considered to be 0.06% butit normally runs from about 0.03% to a trace. The 0.6% soluble iron ismore than enough to control the 0.06% maximum of chloride ion. If ironis not present in the starting sylvite, it may be added artificially atthe bottom of column 11.

The recycle of a strong HNO3 is carried out in either of two ways. Ifraw mother liquor is recycled to the reaction column, then impuritiesare a consideration in the reaction column 11, particularly wherecommercial muriate of potash or sylvinite is the starting material.However, it is preferred to distill the raw acid mother liquor andreturn clean HNO3 only to the reaction column. If this is done, thebuild-up of impurities need only be considered in the smaller recyclepath of the acid mother liquor as will be noted. The recycle of strongHNO3 is only made possible by the production of raw acid mother liquorof greater strength than normal azeotropic acid which is an essentialfeature of this invention.

The 58% HNO3 from the reactor column bottom is taken to the waterremoval column 26 where the water formed in the various reactions orotherwise introduced into the system is removed overhead by vaporizationand the bottoms concentrated to about 80% HNOS or higher (in H2O).Recycle acid mother liquor lsaturated at high temperature with KNOBand/or containing solid KNO3 at lower temperatures may be added to thecolumn 11 bottoms to obtain the desirable KNO3 concentration on the feedplate. The overhead vapor, generally containing about 0.1% HNO3 iscondensed in a direct contact condenser, and demineralized water alongwith wash water from the filtration system is used for reflux. Causticis injected in the direct contact condenser to neutralize the 0.1% acid.The 58% HNO3 (containing dissolved KNO3) is introduced at point 27intermediate the ends of the column. The action in this column iscontrolled largely by the point of entry and concentration of the acidat this point because there is considerable difference in the columnaction above (fractionation) and below (stripping) the point of entry.

The 58% HNO3 on the entry plate has KNOg dissolved therein which hasraised the azeotrope point considerably above 68%. Accordingly vaporsfrom the entering liquor will be considerably below 58% HNOS, e.g.,about 28% HNO3 in the vapors if the solution contains about 80 partsKNO3 and 10 parts NaNO3 per 100 parts solvent (NaNO3 reverses effect ofKNO3 to small degree), as the remaining solution tends to reach thehigher than normal azeotrope point on evaporation. These vapors thuscondense on the next upper plate at 28% acid, but with no KNO3 insolution so that the acid on the upper plate and any thereabove have anazeotrope of 68%. The vapors from this condensed 28% clean acid will beabout 5% HNO3 and, of course, condense on the next plate. This continuesuntil the overhead vapor is only 0.1% HNO3 (or less, depending on columndesign), a part of which may be used as reflux instead of thedemineralized H2O mentioned above. The 28% HNO3 vapors from the feedplate are to be compared with about 40% HNO3 vapors that would resultfrom a 58% HNO3 solution in H2O without any other dissolved materialtherein.

Thus the presence of KNO3 in the HNOS-H2O mixture permits a greaterinitial cut of water in the vapors and would be useful even ifconcentration of a weak acid to a stronger weak acid were the only goal.

When the 58% HNO3 solution passes downward to the next plate, it will beconsiderably more concentrated, e.g., 65% as the result of the weakvapors having been removed. As relatively weak vapors are continuallybeing removed, the concentration of HNO3 gets higher and higher and goesabove the normal 68% HNO3 azeotrope, permitted as the result of the KNO3in solution. However, the bottoms cannot exceed the new or adjusted HNO3azeotropic concentration which depends on the amount of KNOain solution.Generally with 58% acid feed and about 70 to 80 parts KNO3 per 100 partssolvent, the bottoms are about 80% HNOS in the solvent portion. Theadjusted azeotrope as the result of the presence of KNO3 in the solutionpermits the first evaporation at the entry plate to be high in H2O andalso the bottoms to be greater than 68% HNO3. The solution introduced atpoint 27 accordingly must be such that the vapor yielded therefrom willhave at least `slightly less HNO3 than the normal azeotrope of HNO3 andH2O to thus permit fractionation of H2O from the column. Inasmuch asKNO3 is in solution, the solution has a higher HNO3 azeotrope and theHNO3 concentration may be above normal azeotrope but yield a vapor ofless than normal azeotrope and the vapor from the entry plate is thecontrolling factor. For example, an HNOB in H2O with about parts KNO3per 100 parts solvent gives a vapor of about 76% HNO3. If this werecondensed as clean 76% HNOS on the first plate above the feed plate, thevapors from the 76% HNO3 (now clean with no KNO3) would be even higherin HNO3 as the clean strong HNO3 solution would tend to go downwardtoward the 68% HNOS- H2O azeotrope, and thus remove acid instead ofwater. The recycle of mother liquor to column 26 as shown on the drawingmay be introduced anywhere between the bottoms of columns 11 and 26where KNOB is in solution to give more eflicient stripping by raisingthe ratio of KNO3 to HNO3 on the trays which raises the azeotrope andfavors H2O in the vapor composition.

The water removal column could ea-sily be divided into two separatecolumns of units representing the upper fractionation and lowerstripping sections as long as there was passage from the bottom of theformer to the top of the latter. With separate heating means for thedivided units, there would be greater flexibility in operation.Obviously, the reflux ratio coming into the stripping section must bekept a-s low as possible .to increase the acid strength at the bottom.

The 80% HNO3 from the water removal column bottoms is fed at 290 to thecrystallizer-evaporator 29 which has an upper vacuum flash chamber 31,operated under vacuum to give a temperature of about to which the watercolumn bottoms are fed, and a lower atmospheric suspension chamber 32from which crystals in a :slurry are drawn off the bottom as they settlewhile mother liquor is recycled from near the top to the upper chamber.Overhead vapor (somewhat less than the feed e.g., 75% HNOS) from thecrystallizer is condensed in a surface condenser 33, and a secondaryvent condenser is used to reduce acid losses. A two-stage jet andbarometric condenser system provides vacuum for the system. The crystalsare thus created by both cooling and evaporation and their size isbetter than 80% plus 20 mesh. The condensed clean 75% acid is fed to thegas reaction zone 14 near the bottom at a point where the acidconcentration in the column is at or about 75%.

There is a secondary suspension of ne salts in the crystallizer which isremoved as an overflow stream and contains NaNO3, KNO3 and sometimesMg(NO3)2, Ca(NO3)2 and insoluble impurities (depending upon amount ofimpurities bleed and whether the salts at this point exceed theirsolubility), all of which are pumped to a settling cone. The controlledcrystallization forms big crystals of KNO3 by growing from small seednuclei to large crystals. The temperature differential across chamber 31controls the formation of KNO3 seed nuclei. However, since the NaNO3,Mg(-NO3)2, and Ca(iNO3)2 are in such relatively small proportions toKNO3, there is no appreciable growth and t-hese crystals may be removedlargely as fines in the overflow stream.

The liquid from the cone is recycled to the crystallizer and thethickened slurry flows by gravity to an agitator tank 34 where 75% HNO3is added to dissolve KNO3 nes. It may be desirable to filter beforedissolving the fines to take off the mother liquor. The dissolving ofKNO3 from NaNO3, Ca(NO3)2, and Mg(NO3)2 is made possible by the extremesolubility of KNOB in strong HNO3 relative to the much lower solubilityof NaNOa (see FIG. 3). Ca(NO3)2 and Mg(NO3)2 are not shown in the figurebut are not more soluble than NaNO3. The resulting slurry is filteredand the cake removed at 35 as prod-uct (if sufficient NaNO3, it may beprocessed for recovery) or alternatively discarded. The Fe level hasbeen concentrated to about 0.7% (range from 0.5 to 3.0%) in the waterbalance column and the crystal- IK-NO3 fines at agitator tank 34).Ywhich is saturated with KNO3 has reached an acid con- 1.1 lizer. Largecrystals are removed continuously from the crystallizer and passedthrough filter 36 Where they are rwashed With H2O to a low level HNC3content. A liquid bleed 40 is taken I(and used as product, if desired)preferably immediately after filtration to control impurities in thecirculating mother/liquor as the KNO3 is at its lowest point relative toimpurities.

The acid motHer liquor which is saturated with KNO-3 at 150 may berecycled to the reaction column 11, as pointed out before, or may betreated in one or both of two methods (along with 75% acid used todissolve This mother liquor centration of about 83-87% and is taken tovaporizer 38 from which about 81-85% HNO3 vapors will be boiled olf atreduced pressure and about 150-1 60 (or at atmospheric pressure at about220) leaving a slurry of KNO3 (or a rich -solution at 220) which is thenrecycled to the water removal column 26. Evaporators 31 and 38 couldbereversed or combined into a single evaporation step lbut the twostages as shown are preferred. In operating 38 at a considerably highertemperature, a more concentrated HNO3 is obtained but the corrosionproblem is also greater at the higher temperature. The temperature ineither instance is generally at the boiling point ywhich is controlledby the operating pressure.

The 85% HN103 vapors are passed to a condenser 39 and then a portion ofthe condensed acid to heater 43 to form vapors. Then vapors and liquid85% HNO3 are `fed to the reaction column 11 as previously described.

will generally be recycled to storage tank 7, Water removal 'column 26,or column reactor 11, although it may be utilized at various otherpoints in the process. The condensate from 39 may be completelyvaporized by heater 43 and pass only -vapors to reaction column 11 orfractionator 41, in either case, the heat of the vapors vbeing used tofractionate .HNO3 in the respective column.

It will, of course, be appreciated that production of 100% HNO3 is analternative in the processy and depending upon relative demand forstrong HNOS and "KNO3 it is possible to produce from the process onlystrong HNOS, only KNO3, or both, and in fact, a solution of KNO3 in HNO3may also be produced if desired. Such a solution would be particularlyuseful in fertilizer applications, as it would provide nitrate,potassium and aciclulation value. If strong HNO3 is to be withdrawn fromthe cycle as product, a corresponding amount of weak HNO3 could beintroduced most anywhere in the process but preferably at 56 to'thecolumn 11 bottoms. Moreover, it naturally follows that the sub-processesfor producing each of these compounds may be practiced apart from theoverall process in its entirety, e.g., 100% HNO3 may be produced byadding KNO3 to dilute HNO3 and following the process from the waterremoval column 'to the fractionator 41, with recycle of the KNO3.

The solids from ilter 36 are discharged to repulp tank V37 Where theyare slurried with neutral liquor (saturated with KNOB and containingdissolved impurities) and suicient dilute alkali (preferably NaOH) forneutralization of small amounts of residual acid. This slurry is pumpedto a screen 42 for separation of the plus 20 mesh for agriculturalproduct and minus 20 mesh for industrial product. The division by sizinghas no bearing particularly on KNO3 purity but is done becausefertilizer producers want coarse material, whereas industrial KNO3 ispreferred as a finer material.

The agricultural product is filtered and washed to 0.5% NaNO3 withsaturated KNO3 liquor, then dried to about 0.3% moisture content andconveyed to storage. The

agricultural product can run up to 99% IQNOa with about 0.6% NaNO3 andmiscellaneous impurities.

The industrial product is filtered and washed to 0.1% NaNO3 andmiscellaneous impurities, dried to about 0.3% moisture content andconveyed to storage, Thus the industrial product is 99.6% KNO3 and 99.9%KNO3 if H2O is disregarded.

Obviously, the end product may be proportioned differently betweenindustrial and agricultural grades, and, in fact, may be all one or theother. The crystallizer can be adjusted to secure iiner material forindustrial grade or the nes may be recycled to secure a higherpercentage of coarse agricultural grade.

As an alternative, the crystals may be removed only at the bottom ofsuspension tank 32 (i.e., without an overflow removal of lines), inwhich case there will be from 3 to 5% NaNO3 therein and some entrainedHNOS. This will be slurried and passed directly to repulper 37 and theentrained acid neutralized with NaOH to form additional NaNO3. Thisslurry can be passed to screen 42 and the oversize will have suiiicientpurity for agricultural product. The undersize will be taken to asettling cone from which the thickened slurry is removed and passed to abase exchange dissolver where KCl is added, whereby the NaNO3 reacts to,form NaCl and KNO3. The resulting solution is filtered to remove anysolid impurities. The solution is evaporated at any elevatedtemperature, at which point KNO3 is much more soluble than NaCl and thelatter is precipitated at the high temperature. The mixture is ilteredwhile hot water added `to the filtrate and the filtrate cooled, KNO3crystals thus yforming but NaCl remaining in solution.

rNaNO3. This is possibly marketable per se as a fertilizer.

The solution could be chilled to 0 to precipitate the KNO3 and NaNO3 asonly 4% and 3%, respectively, would remain in solution, leavingprimarily NH4NO3.

FIG. 2 FLOW The description heretofore of the flow demonstrated by FIG.1 is primarily for a KCl starting material with relatively small amountsof NaCl as impurity. If, however, there were substantial quantities ofNaCl in the starting material, e.g., sylvinite, it would be desirable tofollow the liow sheet as outlined in FIG. 2. As shown in FIG. 3, thesolubility of NaNO3 is much less than KNO3 in hot, strong HNO3.Therefore, there is apt to be a slurry with solid NaNO3 in the waterremoval column if substantial NaCl was present in the starting material.Accordingly, column 11 bottoms are passed to the .water removal column45 which has above the entry point a top column portion 44 similar tothe top portion of the water removal column 26. Below the point ofentry, however, is a crystallizer-evaporator having an upperilashchamber 48 and a lower suspension chamber 49. Overhead vapors pass intothe fractionation column 44 while va slurry of NaNOa crystals is strongHNO3 is removed at the bottom. Mother liquor from the suspension chamber49 is recycled to the entry point. V ariousother units could be usedbelow the fractionator portion 44 as long after which the filtrate ispassed to evaporator 38. Strong acid vapors would be taken from herejust as in FIG. 1 for recycle to column reactor 11 and/or fractionator41.

The liquor from evaporator 38 will be strong HNO3 slurry of NaNO3crystals and KNO3 in solution almost to saturation. The slurry is passedto filter 46 for removal of additional NaNO3 and the filtrate passed tothe crystallizer evaporator 29 for the controlled crystallization ofKNO3. NaNO3 fines and other fine impurities would be taken off the topof chamber 32 as before but this time recycled to evaporator 38. TheKNO3 crystals would be withdrawn and filtered at 36 and treatedthereafter as before. The mother liquor, saturated with KNO3, willpreferably be recycled to the water removal column.

Various ways of separating KNO3 from NaNO3 will become apparent due tosolubility differences both as a result of temperature of the solute andconcentration of HNO3 in the solute, eg., the solution might be quicklycooled between the reaction column 11 bottom and entry into the waterremoval column and the resultant precipitated NaNO3 removed.

If there is no KCl in the starting chloride material, the

'evaporator crystallizer 29, lter 36 and filter 46 will be eliminated sothat the liquor or slurry will be recycled directly after evaporator 3Sto column 11 bottoms (as shown in dotted line). In such a system,however, it will be necessary to add KNO3, CsNOa or RbNOS to thesolution before entry into the water removal column 26. Such a nitratewould be recycled by the liquor return to column 11 bottoms and entry ofa small amount thereof to provide for processing losses could be made atany point in the cycle and shown here at point 47. It is, of course,essential that KNO3, CsNO3 or RbNO3 be in the solution where HNOS is tobe concentrated above the normal HNOS-H2O azeotrope. Whereas KNO3, CsNO3or R'bNOa is an essential lfeature of the invention, NaNO3 is not andthus if high grade KCl is used without any NaCl, the process will followthrough practically the same as shown in FIG. l but without the removalof NaNO cake.

FIG. 3

FIG. 3 is a graph demonstrating the saturated simultaneous solubility ofKNO3 and NaNOg in varying concentrations of HNO3-H2O solvent at varioustemperature levels. The solubility of NaNO3 rapidly decreases in greaterconcentrations of HNOS and levels off at above about 70% HNO3 at lessthan 20 parts per 100 solvent, regardless of temperature, whereas thesolubility of KNO3 decreases slowly in increased HNOS concentration fora certain range, depending on the temperature, and thereafter thesolubility rapidly increases as 100% HNO3 iS approached.

FIG. 4

FIG. 4 is a graph demonstrating the effect of KNOg in solution on theazeotrope point of the HNO3-H2O solvent. Azeotro-pe point as used hereinmeans the HNOS concentration of the HNO3-H2O solvent at the constantboiling point of a given mixture which may have other compoundsdissolved in the HNO3-H2O solvent. Thus, the normal azeotrope point ofHNO3-H2O only at atmospheric pressure is about 68% HNO3. NaNO3 in thesolution will lower the azeotrope point and when presen-t with KNOBpartly cancels the effect of the KNO3. The effect of KNO3 alone and inconjunction with 10% NaNO3 has been iplotted at about 600 mm. of mercurypressure, which shows the lowering effect of NaNO3. At 35 mm. pressure,the example with NaNO3 present falls below the straight line drawnlthrough the points Without NaN03. The 140 B.P. at 35 mm. pressurerepresents a solution of HNOS-H2O saturated with both KNO3 and NaNO3 `sothat the 35 mm. line is not plotted even in dotted lines beyond 100part-s of KNO3 in the solvent.

At high HNO3 concentrations, only relatively small amounts of NaNO3 aresoluble, as will be noted from FIG. 3. In any event, even if NaNO3 werepresent in amounts equal to KNOa, the lowering effect of NaNO3 does notequal by far the raising effect of KNO3. As shown in FIG. 4 (-in dottedlines representing laboratory work not reduced to specific results),HNO3-H2O saturated with KNO3 can have an azeotrope point of better than90% HNO3, the ceiling having not yet been determined, depending on theboiling point, which controls the solubility of KNO3, and is a functionof the pressure. At any given concentration of KNO3, the `azeotrope willbe higher yas the pressure is reduced. However, in view of the fact thathigher pressure causes higher boiling points and thus higherconcentrations of KNO3 to reach saturation, the ultimate high azeotropeobtainable is at higher pressures.

By distilling HNO3 and H2O at a pressure and temperature and from apoint in the [process such that the liquor is nearly saturated or atleast rich with KNO3, CsNOs or RbNO3, there are produced some totallyunexpected benets. One skilled in the art would assume that such watersoluble nitrates would decrease the vapor pressure of water over aboiling solution of HNO3, H2O and the nitrate. One would, therefore,expect that the vapors would be richer in HNO3 at' the same temperatureand pressure than they would be if no such nitrate were dissolved in theliquor; As a matter of fact, the vapors are substantially richer inWater than they would be otherwise. A definite theory to lexplain thisbehavior is not known, but it may be that there is a tendency for thesespecific nitrates and HNO3 to associate in the rich solutions.

There are many incidental benefits to utilizing this unique property.For one thing, less HNO3 has to be distilled or evaporated to remove aunit quantity of water for water balance. rlherefore, less heat energyis required to remove a unit quantity of water since less latent heat isprovided for HNO3. Fractionation is also easier since water can beremoved from a more dilute solution with fewer plates in thefractionator (or less reflux).

One of the best effects, however, is in increasing the concentration ofacid in the acidic mother liquor which is used for recycle per se orafter distillation. In simple HNOS-H2O solutions, the highestconcentration which can be attained by simple evaporation is theazeotropic mixture (about 68% lat atmospheric pressure). Because of theeffect of the specified nitrate and its tendency to bind the HNO3 insolution and reduce its vapor pressure, however, much higher HNO3concentrations can be produced. Depending upon the amount of HNO3 andH2O in the original reactor bottoms (and this can be varied at will bythe amount of excess acid used with the recycle), the HNOg concentrationin the mother liquor may go easily as high as 85% or even higher. Thisincreased acid concentration at above normal azeotrope has a verybeneficial effect in the column reactor since reactor is a function ofthe concentration of acid used. Recycling of this very strong `acidmother liquor or fresh acid distilled therefrom, allows maintenance ofhigh concentrations in the reaction column and to thus produce morecomplete conversions, particularly lof NOCI and more rapid reaction.

FIG. 5 FLOW Ordinary commercial 32% HC'I (may be any concentration) isstored at 60 and fed into solution tank 9 along with HNO3, the source ofwhich is by alternate routes to be discussed later. The combinedHC1-HNO3 solution from tank 9 is fed intermediate the ends at point 12into the column reactor 11. The reaction gases from tank 9 areintroduced slightly above the liquids `at point 13. Clean HNO3 frompartial condenser 61 is introduced into zone 14 at entry point 68 forthe liquid and at 69 for the vapors. The treatment and action ofoff-gases from zone 14 through Column 21 are identical to that in FIGS.1 Iand 2 which will not be repeated here.

The solution is withdrawn from the bottom of column reactor 11 largelyas a 60% HNO3 solution substantially The bottom-s of column 26 are fedto a non-salting evaporator 64 which produces clean strong HNOS vaporsoverhead and strong HNOS solution containing dissolved KNOS as bottomsfor recycle to provide KNOS on the entry plate of column 26. Thepreferred method is that shown 'in dash lines Where the mother liquor isreturned to solution tank 9 at point 63 as the KNOS in solution has someeffect to lessen corrosion through the system. In this event, the cleanweak HNOS from tower 22 is recycled into the system at point 66 justprior .to column 26. However, if the KNOS-HNOS solution -is recycled at62 (as shown in dotted lines) to the water removal column, then theclean weak HNOS from tower 22 is taken It-o the solution tank 9 at point67 (as shown in dotted lines). In other words, the entry of theKNOS-HNOS solution and the clean weak HNOS may be reversed and obviouslyother :points of entry or combinations may be utilized, bearing in mindthat HNOS from some source is essential in column 11 and KNOS i-sessential in the lower stripping sect-ion of column 26.

FIG. EXAMPLE 1 1n the preferred ow shown, i.e., where the KNOS- HNOSsolution is recycled to the solution tank 9 as shown in dash lines, 100parts HCl (plus 210 parts H2O) at 32%, 440 parts HNOS (plus 110 partsH2O) at 55% and 1100 parts KNOS were fed to tank 9 and passed to column11, resulting in 65 parts Cl2 and- 60 parts NOCI passing into zone 14.440 parts of HNOS (110 parts H2O) at 55 were added to zone 14 whichresulted in 97 parts of C12 and 126 parts of NO2 passing over asolf-gases and 325 parts HNOS (75 parts H2O) at 72% flowing down to zone16. 710 parts HNOS 4.(480 parts H2O) at 60% and 1100 parts KNOS passedfrom column 11 to column 26 to which was added 172 parts of HNOS (140par-ts H2O) at 55% at point 66. From the bottom of column 26 toevaporator 64 was passed 880 parts HNOS (220 parts H2O) at 80% and 1"100 parts KNOS. From the top of evaporator 64 passed thel 440 partsHNOS and 110 parts H2O fed into zone 1 4 and from the bottom the 440parts HNOS, 110 parts H2O and 1100 parts KNOS fed into tank 9 at 67.

FIG. S-EXAMPLE 2 In the other owmodification shown, i.e., where theKNOS-HNOS solution is recycled to the water removal column as shown indotted lines, 100 parts HC1 (210 parts H2O) at 32% and 172 parts HNOS(140 parts H2O) at 55% were fed to tank 9v and passed to column 11,resulting in 65 parts C12 and 60 parts NOCI passing `into zone 14. 970parts HNOS (240 parts H2O) at 80% were added to zone 14 which resultedin 97 parts C12 and 126 parts NO2 passing over as off-gases and 850parts HNOS (260 parts H2O) at 77% flowing down to zone r16. 970 partsHNOS (645 parts H2O) at 54% passed from column 11 to column 26 to whichwas added 970 parts HNOS (240 parts H2O) at 80% and 2400 parts KNOS atpoint 62. From the bottom of column 26 to evaporator 64 was passed 1935parts HNOS (485 parts H2O),at 80% and 2400 parts KNOS. From the top of4evaporator 64 passed the 970 parts HNOS (240 parts H2O) at 80% whichwas fed into zone 14 and from the bottom the 970 parts HNOS (240 partsH2O) at 80% and 2400 parts KNOS fed into column 26.

FIGS. 6 TO 9 As hasbeen pointed out in regard to previous gures, it ispossible to merely concentrate HNOS as the main yproduct and ignore theconversion of the various chlorides. Furthermore, it has been found thatany nitrate salt of the metals in group Ia having an atomic weightscribed in FIGS. 1 to 5. Moreover, solutions of other metal nitratesthat behave as dehydrating agents may be used in alternate steps inconjunction with the above defined nitrates to aid nitric acidconcentration.

FIG. 6 FLOW As shown in FIG. 6, fresh weak HNOS of 55 concentration isstored in tank 600 and solid KNOS in bin 70 and both are fed intodissolving tank 80 in the proper amounts to maintain about 50 parts KNOSto 100 parts solvent in the dissolving tank 80. After mixing anddissolving the solution from dissolving tank is fed to the waterstripper column for boiling. The overhead vapors from stripper 90 arepassed to water rectifier column wherein fractionation occurs to give anoverhead of less than 1% HNOS and a bottom stream of about 65% HNOSwhich is cycled back to the dissolving tank 80.

The effectiveness of column 100 is inliuenced by the concentration ofacid at the point of entry into column 90 (this point of entry would bethe intermediate feed point if columns 90 and 100 were combined) becausethere is considerable difference above (fractionation) and below(stripping) the point of entry 130. The functions of columns 90 and 100can be accomplished in one column by feeding intermediate the columnends, but it is preferred to have two columns and select differentmaterials, sizes, rates, and other conditions according to the differentcircumstances presented. It will be appreciated that the liquid incolumn 100 is clean HNOS- H2O and is merely being fractionated, whereasthe liquid in column 90 is a solution of KNOS in the HNOS-H2O solventwhich is being boiled to concentrate the HNOS and the KNOS.

The 55% HNOS at the entry point has KNOS dissolved therein which hasraised the azeotrope point considerably above 68%. Accordingly, vaporsfrom the entering liquor will be considerably below 55% HNOS, e.g.,about 28% HNOS in the vapors if the solution contains about 50 partsKNOS per 100 parts solvent, as the remaining solution tends to reach thehigher than normal azeotrope point on evaporation. These vapors thuscondense on the next upper plate (i.e., in column 100) at 28% acid butwith no KNOS in solution so that the acid on any plates in column 100has an azeotrope of 68%. The vapor from this condensed 28% clean acidwill be about 5%-HNOS and, of course, condense on the next plate. Thiscontinues until the overhead vapor is only 0.1% HNOS (or less, dependingon column design) a part of which may be used as reliux if desired. The28% HNOS vapors at the feed plate are to be compared to about 42% HNOSvapors that would result from a 55 HNOS solution in H2O without anyother dissolved material therein. Thus, the presence of KNOS in theHNOS-H2O mixture permits a greater initial cut of water in the vaporsand would be useful even if concentration of a weak acid to a strongerweak acid were the only goal.

When the 55 HNOS solution passes downwardly to the next plate in colurnn90, it will be considerably more concentrated, eg., 63% as the result ofthe weak vapors having been removed. As relatively weak vapors arecontinually being removed, the concentration of HNOS gets higher andhigher and goes above the normal 68% HNOS azeotrope permitted, as aresult of the KNOS in solution. However, the bottoms cannot exceed thenew or adjusted HNOS azeotropic concentration Which depends upon theamount of KNOS in solution. Generally, With 55 acid feed and about 50parts KNOS per 100 parts solvent, the bottoms are about 80% HNOS in thesolvent portion and 65 parts KNOS per 100 parts solvent. The adjustedazeotrope as a result of the presence of KNOS in the solution permitsthe irst evaporation at the entry plate to be higher in H2O and also thebottoms t0 be greater than 68% HNOS.

The solution introduced at the entry point 130 accordingly should besuch that the vapor yielded therefrom will have at least slightly lessHNO3 than the normal azeotrope of HNO3 in H2O to thus permitfractionation of H2O from the column 100. Inasmuch as KNO3 is insolution, the solution has a higher HNO3 azeotrope, and the HNO3concentration may be above the normal azeotrope but yield a vapor ofless than the normal azeotrope and the vapor from the entry plate is thecontrolling factor. For example, an 80% HNO3 in H2O with about 100 partsKNO3 per 100 parts solvent gives a vapor of about 76% HNOS. If this werecondensed as clean 76% HNO3 on the iirst plate above the feed plate, thevapors from the 76% HNO3 (now clean with no KNO3) would be even higherin HNO3 as the clean 'strong HNOS solution would tend to go downwardtoward the 68% HNO3 H2O azeotrope, and thus remove acid instead ofwater.

The bottom stream from stripper 90 is now about 80% HNOS to 20% H2O andabout 65 parts KNO3 to 100 parts solvent, the acid concentration of thesolvent having gone over 68% in column 90 vonly because of the presenceof KNO3 in the solution.

The bottom stream from column 96 is fed to evaporator 110 where strongacid vapors yof about 80% HNO3 are removed from the top and fed tofractionating column 120. The bottom stream evaporator 116 is a slurryof KNO3 crystals in about 85% HNOS solvent and is recycled to dissolvingtank 80. The fractionator 120 may be operated to remove up to 100% HNO3(generally 95-100%) as the overhead stream as product and the bottomstream down to the normal azeotrope of about 68% HNO3 which is alsorecycled to dissolving tank 8i).

As shown in FIG. 6, the only withdrawals from the system are H2O fromthe top of column 160 and HNO3 from the top of fractionator 120. It isobvious, of course, that withdrawals can be made at many other points,particularly the strong clean HNO3 from the top of evaporator 110 tothus eliminate fractionator 120, or the various recycles may bewithdrawn or recycled to Iother points. As a practical matter, however,the recycle of KNOB permits operation of the system Iby the additiononly of outside weak HNO3 except for replacement of processing losses inKNOB.

FIG. 9

The specic example described heretofore is concerned with potassiumnitrate but largely the same results are `obtained with both cesium andrubidium nitrates as will more specifically be noted from thevapor-liquid equilibrium diagram of FIG. 9, wherein the results of 50parts of the three nitrates dissolved in 100 parts of HNO3-H2O mixtureof 620 mm. pressure are shown in comparison with HNO3-H2O mixturewithout a metal nitrate therein.

TABLE I As was previously known in the art, several compounds, includingvarious nitrates (e.g., NaNO3, Ca(NO3)2 and Mg(NO3)2), reduce theazeotrope point of nitric acid, i.e., these compounds tend `to hold morewater in the solution and thus permit greater evaporation of nitric acidand one would expect that the same'would be true of all nitrates. Inorder to explain a possible theory of the present discovery variousmetal nitrates have been studied and considered. It would appear thatalkali metal nitrates (i.e., group Ia of the periodic table) having anatomic weight as heavy or heavier than potassium will increase theazeotropic point, i.e., nitrates of potassium, rubidium, cesium andfrancium which are included within the scope of this invention. Thefollowing table sets forth the results noted on the azeotrope of HNO3with 50 parts of various metal nitrate salts in solution per 100 partsof solvent.

Thus it will be noted that silver nitrate has practically no effect andthat sodium nitrate has only a slight elect but opposite to the nitratesof this invention.

It is believed that the relationship of the ionic radius to the charges(valence) on the metal atom determines the effect of the specilicnitrate on the azeotrope point. Accordingly, the above table includesthe ionic radius of the various metals and the ratio of the charge tothe radius. Using this approach it will be observed that the results ofthe tested nitrates not only can be predicted but largely the degree ofthose results. Accordingly, all

untested metal (except francium) nitrates will reduce the HNOSconcentration at the azeotrope.

FIG. 7 FLOW The majority of the nitrates (as shown in Table I) reducethe amount of HNO3 in the azeotrope and it has been found that these`azeotrope lowering nitrates may be used in combination with theazeotrope raising nitrates to produce strong HNO3. In FIG. 7, theprocess is the same as FIG. 6 from dissolving tank 80 through evaporatorso that the drawings and reference numerals are identical. However, theoverhead vapors from evaporator 110 maybe passed into dehydrator 140Where they are contacted with .a down-flowing solution of Mg(NO3)2whereby an even stronger HNO3 vapor is removed as overhead and -passedto fractionator 160 if desired to increase the HNO3 even higher. Thefractionator bottoms (about 68% HNOa) are recycled to the dehydrator.The bottoms from dehydrator are concentrated with Mg(NO3)2 in evaporator180 by removing weak HNO3 overhead which is recycled to dissolving tank80. The 'bottoms from evaporator 180 containing a very highconcentration of magnesium nitrate are recycled to the top of dehydrtor140 to provide the Mg(NO3)2 for dehydrator 4 FIG. 8 FLOW An alternatecombination is shown in FIG. 8 wherein a weak HN O3 solution containingCsNO3 is fed to evaporator 200. The evaporation is controlled so thatthe overhead vapors which are fed to rectifier 220 are below the normalazeotrope of 68% HNOS. Water is removed by rectifier 226 and the bottomsof about 68% HNO3 are recycled to evaporator 200. The bottoms fromevaporator 266 are taken to evaporator 240 where the bottoms of veryhigh CsNO3 concentration are recycled to evaporator 200. The evaporationis controlled so that the overhead vapors from 240 are above or near 68%HNO3 which are fed into evaporator 260 that contains a highconcentration of Mg(NO3)2. This, of course, lowers the azeotrope pointto about 20% HNO3 and so the overhead vapors from this solution are muchhigher (and thus above 68%) in HNOS than the bottoms. The overheads aretaken to a fractionator 280 and the bottoms are concentrated inevaporator 300 by removing near 68% HNO3 overhead for recycle toevaporator 200. The bottoms from evaporator 300 is high in Mg(NO3)2 andlow in HNO3 and is recycled to evaporator 260.

Other alternatives that use the combination of azeotrope raisingnitrates (Cs, Rb, K) and azeotrope lowering nitrates (Li, Ca, Cu, Mg,Fe, Al) will be obvious to those skilled in the art in light of thisdisclosure.

We claim:

1. A method of concentrating nitric acid in a water solution comprisingdissolving in said solution a nitrate of a group Ia metal having anatomic weight at least as heavy as potassium and thereafter vaporizingsaid solution to remove vapors therefrom.

2. A method of producing strong nitric acid from a weak nitric acidsolution comprising dissolving in said weak nitric acid solution anitrate of a group Ia metal having an atomic weight at least as'heavy aspotassium and thereafter vaporizing said weak nitric acid-metal nitratesolution to remove vapors therefrom until the remaining solution is saidstrong nitric acid, said strong and said weak acids having acidconcentrations respectively greater and lesser than the normalazeotropic mixture of nitric acid and water only.

3. A method of producing distilled strong nitric acid from a weak nitricacid solution comprising dissolving in said weak solution a nitrate of agroup Ia metal having an atomic weight at least as heavy as potassium,fractionally distilling water vapors overhead from said weak nitricacid-metal nitrate solution until the remaining solution is strongnitric acid, and removing vapors from said remaining solution, wherebysaid vapors are said distilled strong nitric acid, said strong and saidweak acids having acid concentrations respectively greater and lesserthan the normal azeotropic mixture of nitric acid and water only.

4. The method of claim 3 wherein said metal nitrate is potassiumnitrate.

5. A method of producing a second strong nitric acid from a iirst weaknitric acid solution comprising dissolving in said first weak nitricacid solution a nitrate of a group Ia metal having an atomic weight atleast as heavy as potassium, boiling said Weak nitric acid-metal nitratesolution, recovering the vapors from said boiling until a first strongnitric acid solution remains, fractionally distilling said recoveredvapors to remove water vapor overhead and recover as bottoms a secondweak nitric acid, recycling said second weak nitric acid to said firstweak nitric acid solution, distilling said rst strong nitric acid torecover distillate and residue, said distillate being said second strongnitric acid, said residue including said metal nitrate and a thirdstrong nitric acid, and recycling said residue to said rst weak nitricacid solution, said strong and said weak acids having acidconcentrations respectively greater and lesser than the normalazeotropic mixture of nitric acid and water only.

6. A method of removing water in a column from a combined solutionhaving components of nitric acid, water and a nitrate of a group Iametal having an atomic weight at least as heavy as potassium, comprisingintroducing at a point intermediate the ends of said column saidcombined solution with a ratio of components such that the actual vaporsfrom said solution have less nitric acid than azeotropic vapors ofnitric acid in Water only, forming said actual vapors, and fractionallydistilling said actual vapors in said column above said point untilsubstantially only water vapors are removed overhead.

7. A method of producing strong nitric acid from a weak nitric acidsolution comprising dissolving in said weak solution a nitrate of agroup Ia metal having an atomic weight at least as heavy as potassium,fractionally distilling water vapors overhead from said weak nitric acidmetal nitrate solution until the remaining solution is strong nitricacid, vaporizing said remaining solution to remove vapors of strongnitric acid therefrom, and contacting said strong nitric acid vaporswith a magnesium 'nitrate solution to produce a stronger nitric acidvapor as overhead and weak nitric acid-magnesium nitrate solution asbottoms, said strong and said Weak acids having acid concentrationsrespectively greater and lesser than the normal azeotropic mixture ofnitric acid and water only.

8. A method of reacting a chloride of a alkali metal selected from thegroup consisting of potassium, sodium and mixtures thereof with aqueousnitric acid to produce chlorine, nitrogen oxide, water and thecorresponding alkali nitrate comprising first partially reacting thechloride with a first aqueous nitric acid, thereafter furthering thereaction by contacting strong nitric acid with the reaction vapors, andpassing the acid remaining from the strong nitric acid into the reactionsolution, said strong acid having an acid concentration greater than thenormal azeotropic mixture of nitric acid and water only.

9. A method according to claim 8 wherein said first nitric acid is weak,said weak acid having an acid concentration less than the normalazeotropic mixture of nitric acid and Water only.

10. A method according to claim 9 wherein the alkali metal chloride ispotassium chloride and the nitric acid strength at the end of thereaction is at least 30%.

11. A method according to claim 10 wherein sufficient quantity of saidstrong nitric acid is added to drive the unreacted potassium chloride inthe reaction mixture to substantially complete reaction.

12. A method of reacting a chloride of an alkali metal selected from thegroup consisting of potassium, sodium and mixtures thereof with aqueousnitric acid to produce chlorine, nitrogen oxide, water and thecorresponding alkali metal nitrate comprising first partially reactingthe chloride with a first aqueous nitric acid in a first zone, passingthe reaction gases from the first zone to a second zone, furthering thereaction by contacting strong nitric acid with the reaction gases in thesecond zone, and passing the remaining nitric acid from the second zoneto the first zone, whereby any nitrosyl chloride is oxidized to.chlorine and nitrogen dioxide, said strong acid having an acidconcentration greater than the normal azeotropic mixture of nitric acidand water only.

13. A method according to claim 12 wherein said iirst nitric acid isweak, said Weak acid having an acid concentration less than the normalazeotropic mixture of nitric acid and water only. i

14. A method according to claim 13 wherein at least part of the strongnitric acid is produced from the remaining acid by fractionallydistilling water vapor therefrom in the presence of a nitrate of a groupIa metal having an atomic weight at least as heavy as potassium.

15. A method according to claim 14 wherein said nitrate is potassium andis produced from potassium chloride in the reaction.

v 16. A method according to claim 15 wherein said potassium nitrateforms a slurry and nitric acid vapor is removed from the slurry whilepotassium nitrate crystals are removed from the slurry, the nitric acidVapor being condensed to form said strong nitric acid for recycling.

17. A method according to claim 1S wherein the mixture of liquor and thepotassium nitrate form a slurry and nitric acid vapor is removed fromthe slurry while potassium nitrate crystals are removed from the slurry,the nitric acid vapor being condensed to form an intermediate strongnitric acid from which water is then evaporated to form said strong acidfor recycling to the gas reaction area.

18. A method according to claim 15 wherein the mixture of liquor and thepotassium nitrate form a slurry and nitric acid vapor is removed fromthe slurry while potassium nitrate crytsals are removed from the slurry,the nitric acid vapor being condensed to form a second strong nitricacid for recovery per se.

19. A method according to claim 12 wherein said alkali metal chloride ispotassium chloride and chloride impurities up to 10% of sodium, calciumor magnesium are present whereby a nitrate mixture of potassium andsodium, calcium or magnesium is formed, dissolving said nitrate mixturein a solution, evaporating said solution to supersaturation of saidnitrates, crystallizing said potassium nitrate in an excess of saidsupersaturated solution by growing large crystals on small potassiumnitrate nuclei, whereby said impurities form only small crystals, andseparating said large crystals of potassium nitrate from said smallcrystals by size classification.

20. A method according to claim 12 wherein the said alkali metalchloride is sylvinite whereby a nitrate mixture of potassium and sodiumis formed, dissolving said nitrates in weak nitric acid at aconcentration where the solubility of potassium nitrate is at itslowest, removing vapors from said weak nitric acid until crystallizationof sodium nitrate, said vapor removal increasing the concentration ofsaid nitric acid whereby the solubility of potassium nitrate thereinwill increase and the solubility of sodium nitrate continues todecrease, and separating said sodium nitrate crystals from saidconcentrated nitric acid.

21. A method according to claim 12 wherein the reaction zone is dividedinto at least two areas, the second area being a gas reaction area forfurthering the reaction and the first area being a solution reactionarea for the partial reaction and the strong nitric acid is added in thegas reaction area countercurrently to the reaction gases therein.

22. A process of oxidizing hydrochloric acid to chlorine comprisingreacting said hydrochloric acid with suiiicient excess nitric acid, atleast part of which is a first strong nitric acid, to form a weak nitricacid solution of at least 40% at the end of said reaction, removingnitrogen oxides and said chlorine as gases, fractionally distillingwater vapors from said weak nitric acid solution in the presence of adissolved nitrate of a group Ia metal having an atomic weight at leastas heavy as potassium until the remaining solution is a second strongnitric acid, and recycling at least part of said second strong nitricacid as said first strong nitric acid, said strong and said weak acidshaving acid concentrations, respectively, greater and lesser than thenormal azeotropic mixture of nitric acid and water only.

23. A process of oxidizing hydrochloric acid to chlorine comprisingintroducing said hydrochloric acid and nitric acid into a first zone,introducing first strong nitric acid into a second zone, providing heatto boil any liquids in said zones, passing reaction gases from saidfirst zone into said second zone, exposing said reaction gases in saidsecond zone to said first strong nitric acid to oxidize any nitrosylchloride to chlorine and nitrogen dioxide, passing said exposed nitricacid of said second zone to said first zone, the total amount andconcentration of said nitric acids being sufficient to complete thereaction and to have a weak nitric acid solution of at least 40%concentration at the end of said reaction, passing said chlorine andnitrogen dioxide gases from said second zone, withdrawing the weaknitric acid solution from said first zone, fractionally distilling watervapors from said withdrawn nitric acid solution in the presence of adissolved nitrate of a group Ia metal having an atomic weight at leastas heavy as potassium until the remaining solution is a second strongnitric acid, and recycling at least part of said second strong nitricacid as said first strong nitric acid, said strong and said weak acidshaving acid concentrations, respectively, greater and lesser than thenormal azeotropic mixture of nitric acid and water only.

24. A process of oxidizing hydrochloric acid to chlorine comprisingintroducing hydrochloric acid and nitric acid at the top of a firstcolumn zone, passing reaction gases from the top of said first zone tothe bottom of a second column zone, providing heat to boil any liquidsin said zones, introducing a first strong nitric acid into said secondcolumn zone in sufficient strength and quantity to convert any nitrosylchloride present to chlorine and nitrogen dioxide, said zones formingportions of a functional vertical column with said first zone beingbelow said second zone, the total amount and concentration of saidnitric acids being sufficient to have a weak nitric acid of at least 40%concentration at the bottom of said first zone after completion of thereactions, removing the bottoms of said first zone, fractionallydistilling water vapors from said rst zone bottoms in the presence of adissolved nitrate of a group Ia metal having an atomic weight at leastas heavy as potassium until the remaining solution is a second strongnitric acid, and recycling at least part of said second strong nitricacid as said first strong nitric acid, said strong and said weak acidshaving acid concentrations, respectively, greater and lesser than thenormal azeotropic mixture of nitric acid and water only.

25. The process of claim 24 wherein the nitric acid is at least 72% atthe bottom of said second zone after conversion of said nitrosylchloride.

26. The process of claim 25 wherein said weak nitric acid is at leastabout 57%.

27. The process of calim 24 wherein said metal nitrate dissolved in aportion of said second nitric acid is recycled to said fractionaldistillation step.

28. The process of claim 24 wherein said metal nitrate dissolved in aportion of said second strong nitric acid is recycled to said firstzone.

29. The process of claim 24 wherein said strong nitric acid containingsaid metal nitrate is partly distilled, the distilled portion beingrecycled as said first strong nitric acid and the remaining portioncontaining said metal nitrate being recycled through said fractionaldistillation step.

30. The process of claim 24 wherein said metal is potassium.

No references cited.

MAURICE A. BRINDISI, Primary Examiner.

1. A METHOD OF CONCENTRATING NITRIC ACID IN A WATER SOLUTION COMPRISINGDISSOLVING IN SAID SOLUTION A NITRATE OF A GROUP IA METAL HAVING ANATOMIC WEIGHT AT LEAST AS HEAVY AS POTASSIUM AND THEREAFTER VAPORIZINGSAID SOLUTION TO REMOVE VAPORS THEREFROM.